This invention relates to an axial flow ring fan and in particular to an improvement that increases the fan's operating efficiency and reduces fan noise.
Examples of known axial flow ring fans are shown in U.S. Pat. Nos. 4,358,245 and 4,569,632. The former patent shows a fan in which the blades are forwardly skewed. It is conventional practice to fabricate these fans from injection moulded plastic so that the hub, the blades, and the ring are an integral structure.
The fan of the present invention comprises forwardly skewed blades each of whose leading edge has a somewhat sinusoidal shape when viewed in the circumferential direction and which falls off in the radial direction.
This sinusoidal shape may be defined in terms of varying pitch ratio for the blade along the radial extent of the blade. More specifically, it may be defined in terms of the pitch ratio to average pitch ratio as a function of the blade's non-dimensional radius wherein that characteristic is substantially constant for non-dimensional radii between 0.4 and 0.495, is decreasing for non-dimensional radii between 0.495 and 0.55, is substantially constant for non-dimensional radii between 0.55 and 0.675, is increasing for non-dimensional radii between 0.675 and 0.85 and is decreasing for non-dimensional radii greater than 0.85. The pitch ratio at any particular non-dimensional radius is 6.28 times the non-dimensional radius times the tangent of angle Q where angle Q is the acute angle between a first line extending between the leading and trailing edge points of a planar projection of the cross-section of the blade along the particular non-dimensional radius and a second line that extends through the trailing edge point and is perpendicular to the direction of projection. The average pitch ratio of the blade is an average of the pitch ratios at a number of non-dimensional radii of the blade sufficient to at least approximate the actual average. In the disclosed fan the pitch ratio to average pitch ratio is approximately 1.07 for non-dimensional radii between 0.4 and 0.495, approximately 1.044 for non-dimensional radii between 0.55 and 0.675 and approximately 1.105 at a non-dimensional radius of 0.85.
Fall off in the radial direction is defined by the fall off ratio. The numerator of the ratio is determined by the axial distance between the radially outermost point and the radially innermost point on a blade as taken in a radial cross section through the blade. The denominator of the ratio is determined by the radial distance between those two points.
A fan constructed in accordance with principles of the present invention attains an improvement in axial flow, an improvement in internal operating efficiency, and an attenuation of fan noise with a considerable reduction in rotational noise component leading to an improvement in the tonal quality of the fan. Features of the invention will be described with reference to the accompanying drawings which illustrate a presently preferred embodiment constructed in accordance with the best mode contemplated at the present time for carrying out the invention.